Liquidity is the ability to buy or sell a relevant quantity of a contract quickly and at a price close to the prevailing market level, without causing a disproportionate price move. In futures markets, it is therefore an execution-quality concept: a liquid contract lets participants transfer risk with relatively low friction, while an illiquid contract requires more time, a wider price concession, or both.
Activity is not execution capacity
Trading Volume and Open Interest can provide useful context about market activity and participation, but neither is liquidity itself. Volume counts completed trading; open interest counts outstanding contracts. Liquidity asks a different question: what would it cost to execute now, for the size that matters? A market may trade heavily while its bid-ask spread widens, available depth falls, or orders move the price sharply. During the May 6, 2010 market disruption, CFTC staff noted that exceptionally high E-mini S&P 500 futures volume did not correspond to better liquidity, illustrating why volume can become a misleading proxy under stressed conditions.
Size changes what liquidity means
No single statistic captures liquidity for every order. For a small trade, the bid-ask spread may be the most relevant measure because execution can occur near the best quoted prices. A larger order may consume several price levels, making order-book depth and price impact more informative. A participant working an order over time also cares about how quickly liquidity replenishes after trades. This makes liquidity multidimensional and conditional on order size, trading horizon, contract month, time of day, volatility, and market conditions.
Displayed depth should also be treated as a proxy rather than the complete amount of executable interest. A 2017 CFTC Office of the Chief Economist staff paper examining selected futures markets with data from 2013 through mid-2016 measured public order-book depth from visible posted quantity; hidden liquidity such as iceberg orders was outside that measure, and potential liquidity arising from the individual legs of spread orders was not included. The practical consequence is that two markets showing the same quoted spread or visible depth can still deliver different execution outcomes. Liquidity is best judged from several complementary measures—such as spread, depth, price impact, and trading activity—matched to the size and manner of the trade being evaluated.